Combustion misfires lead to an increase of the toxic substances which are emitted during the operation of the engine and can furthermore lead to damage of a catalytic converter in the exhaust system of the engine. A detection of combustion misfires in the total rpm and load ranges is necessary to satisfy statutory requirements for the on-board monitoring of exhaust-gas relevant functions. In this context, it is known that characteristic changes of the rpm trace of the engine occur during operation with combustion misfires compared to normal operation without misfires. Normal operation without misfires can be distinguished from operation with misfires from the comparison of the rpm traces.
A method operating on this basis is already disclosed in German patent publication 4,138,765, corresponding to U.S. patent application Ser. No. 07/818,884, filed Jan. 10, 1992, now abandoned.
In accordance with this known method, a crankshaft angle region identified as a segment is assigned to a specific region of the piston movement of each cylinder. The segments are realized, for example, by markings on a transducer wheel coupled to the crankshaft. The segment time in which the crankshaft passes through this angle region is dependent, inter alia, from the energy converted in the combustion stroke. Misfires lead to an increase of the segment times which are detected in synchronism with the ignition. According to the known method, an index for the rough running of the engine is computed from the differences of the segment times. Additional slow dynamic operations, such as the increase of the engine rpm during vehicle acceleration, are mathematically compensated. A rough-running value, which is computed in this way for each ignition, is likewise compared ignition synchronously to a predetermined threshold value. This threshold value is, if required, dependent upon operating parameters such as load and rpm and exceeding this threshold is evaluated as a misfire.
The reliability of the method is decisively dependent upon the determination of the segment times and therefore on the accuracy with which the markings can be produced on the transducer wheel during manufacture. Additional sources of error result from the position of the transducer wheel, that is, because of the eccentricity of its arrangement on the crankshaft as well as from the spacing between the transducer wheel and the inductive pickup. These mechanical imprecisions can be mathematically eliminated by an adaptation method carried out during overrun operation of the engine. Such a method is disclosed, for example, in U.S. Pat. Nos. 5,428,991 and 5,696,316. Segment times can be determined in normal operation outside of overrun operation and be logically coupled to the corrective values. Deviations of these segment times are therefore independent of manufacturing inaccuracies of the transducer wheel and point to other causes such as torsion oscillations of the crankshaft.
Torsion oscillations are superposed on the rotational movement of the crankshaft. The torsion oscillations occur primarily at high rpms during fired operation and lead to a systematic lengthening or shortening of the segment times of individual cylinders so that, for example, the detection of misfires is made more difficult. For this reason and for the reason of differences particular to each cylinder because of wear or manufacturing inaccuracies, a base noise in the form of a scattering width of the segment times remains even after the transducer wheel adaptation and this basic noise is not attributable to misfires. Actual misfires can be distinguished from this base noise with greater difficulty the less individual misfires operate on the rpm of the crankshaft. The reliability of the misfire detection therefore drops with increasing number of cylinders of the engine and with increasing rpm as well as decreasing load.
A method is disclosed in U.S. patent application Ser. No. 08/658,507, filed Jun. 10, 1996, wherein the determination of the corrective values takes place during fired operation, that is, during normal operation outside of overrun operation.
This method can always be advantageously utilized (also separately from the misfire detection) when a high-resolution detection of rpm is needed.